Scanning monochromators are used, for example, in the analysis of liquid samples which are contained in glass cells. A light beam of defined wavelength is projected through the cell in which the liquid sample is contained, and the portion of the light beam which passes through the cell is detected by a light detector on the opposite side. There are also many other known uses for scanning monochromators.
To obtain the desired wavelength band of radiation for passing through the liquid sample, a scanning monochromator is used. It typically includes an entrance aperture, rotatable spectral-dispersion element (e.g., a diffraction grating or prism), drive means for rotating the spectral-dispersion element, an exit aperture, and optionally optical elements for collimating the radiation from the radiation source. Radiation dispersed by the dispersion element is imaged at the exit aperture as a narrow wavelength band of radiation. The bandwidth of the radiation depends upon the physical properties of the dispersion element, the focal length of the optical system, the width of the entrance and exit apertures, and the incident and diffracted angles off the dispersion element.
Typically, the dispersion element (e.g., diffraction grating) is driven by a drive motor through a mechanical drive that includes a sine bar and lead screw. There are various disadvantages associated with the use of such a mechanical arrangement, including dirt contamination, need for lubrication maintenance, and mechanical wear.
Although some direct drive systems have been proposed, there are also disadvantages associated with such systems. For example, in U.S. Pat. No. 4,469,441 there is described a galvanometer direct drive system. The disadvantages of such system include hysteresis and the fact that it must utilize a very high resolution input signal to control current to the galvanometer. The drive also has a limited usable range of angular rotation. This limits the wavelength coverage of the monochromator. The drive system is also very temperature-sensitive. Further, use of a worm gear can result in mechanical wear.
U.S. Pat. No. 5,096,295 (Krupa) describes a scanning monochromator which uses a micro-stepping motor to drive a spectral-dispersion element by means of a reduction-gear harmonic drive. The disadvantages of such system include mechanical inaccuracies and wear, and slow response to change from one angle to another angle. Therefore, there is slow response to change the wavelength of the monochromator.
EPO Publication 381053 describes a spectrophotometer which includes a pulse motor to rotate a diffraction grating through a gear-reduction mechanism. This drive system is the same as that described in U.S. Pat. No. 5,096,295.
U.S. Pat. No. 5,015,937 describes a control system for a microstepping motor. This control system doesn't have the ability to position a motor with the resolution and accuracy necessary to control a monochromator. There is no description in the patent as to the manner of programming the pulse width modulator (PWM).
U.S. Pat. No. 4,855,660 describes a control system for a microstepping motor. The motor uses sense resistors to control the motor current. The control system lacks the resolution and accuracy to control a monochromator. The system will also drift in motor position with temperature.
There has not heretofore been described a scanning monochromator having the advantages provided by the present invention.